1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus.
2. Description of the Related Art
Conventionally, magnetic resonance imaging apparatuses (hereinafter, “MRI apparatuses”) that collect data of the internal body of a subject by use of a magnetic resonance phenomenon and reconstruct a magnetic resonance image of the internal body play an important role in various medical practices such as diagnosis, treatment, and surgery planning of diseases.
With the magnetic resonance imaging method executed by an MRI apparatus, a spatial resolution, an imaging time, and a signal/noise ratio of a magnetic resonance image and the like are dependent on imaging conditions such as the field of view (FOV), the imaging matrix, and the number of acquisition, which are parameters of pulse sequences. For this reason, the MRI apparatus is one of medical instruments that require settings for a large number of imaging conditions.
For example, because the sizes of subjects vary, the operator of the MRI apparatus needs to adjust the region of interest (including a slab if an image to be taken is three-dimensional) in accordance with the size and position of the subject at the time of planning the positioning.
When the region of interest to be set at the time of positioning planning is different from the preset field of view, the operator needs to adjust the field of view in accordance with the size of the subject. For example, when the subject is larger than the field of view but the field of view is not large enough with respect to the region of interest, aliasing artifacts may appear in the magnetic resonance image. Then, the operator needs to adjust the imaging region.
When the region of interest is too small for the subject, an image is taken in a large field of view to reconstruct a magnetic resonance image, and then the region of interest is cut out of the reconstructed magnetic resonance image so that aliasing artifacts can be avoided. With such a method, small sampling intervals in the time domain are set to maintain the spatial resolution, and after a Fourier transform is performed, the region of interest is cut out.
However, if a large field of view is set to prevent aliasing artifacts from occurring in the magnetic resonance image and small sampling intervals are set in the time domain to maintain the spatial resolution in the magnetic resonance image, the imaging time is adversely increased. To prevent the imaging time from unnecessarily increasing, the operator needs to determine the minimum field of view in which no aliasing artifact would enter the region of interest.
On the other hand, when the subject is smaller than the field of view, an aliasing artifact would not appear if the region of interest is used as a field of view. However, the imaging time would be increased because unnecessary areas have to undergo the imaging process. In addition, because the unnecessary areas need to undergo the imaging process, the amount of data would be increased. This would especially increase the load on the system for executing the data collecting process and the reconstruction calculating process. Thus, even when the size of the subject is smaller than the field of view, the operator needs to determine the minimum field of view so that the imaging time would not be unnecessarily increased.
To reduce the imaging time, a technique is known with which a direction in which the projection has the minimum width (the minimum size of the subject) is detected from projection data obtained by projecting the subject from multiple directions, and the detected direction is automatically set as a phase encoding direction (see, for example, JP-A H3-16851 (KOKOKU)).
According to the above conventional technique, the imaging time may be reduced, but the relationship between the sizes of the subject and the region of interest is not taken into consideration. Thus, the operator has to determine the minimum field of view in which aliasing artifacts would not enter the region of interest.
In other words, the operator needs to set the region of interest and also to adjust the minimum field of view at the time of planning the positioning. For this purpose, the accurate position of the region of interest in the subject has to be obtained. This means that it takes a lot of time and efforts for the operator to set the minimum field of view. This setting operation requires a long time to learn. Thus, the total examination time tends to be increased.
As discussed above, a problem of difficulty in setting the field of view free of artifacts resides in the conventional technologies.